1. Field of the Invention
The invention relates to a process for the removal of chlorine from off-gases which continuously or sporadically contain small amounts of chlorine, by scrubbing the off-gases with a ferrous chloride-containing aqueous solution in a scrubbing system.
2. Description of the Related Art
Off-gases are formed in many processes, particularly in the production of titanium dioxide by the chlorination of titaniferous and ferriferous feedstock material, thus generating titanium tetrachloride and iron chloride, and by the reaction of the titanium tetrachloride with oxygen-containing gases (chloride process). These off-gases generally contain CO.sub.2, CO, N.sub.2 and often also contain HCl. But these off-gases may further contain small amounts of chlorine either continuously or sporadically. This is frequently the case, for instance, if operating failures are encountered in the chloride process. In the chloride process, this amount of chlorine may, in general, be present in an amount up to two per cent by volume.
For reasons of pollution control, the off-gases must be freed of detrimental constituents before they can be released into the atmosphere. As a rule, this has been done so far by first scrubbing the off-gases with an alkaline solution, for example a sodium hydroxide solution or a sodium carbonate solution and, if necessary, by subsequently subjecting them to an oxidation step to react the carbon monoxide. The alkaline solution absorbs hydrogen chloride and chlorine, if present. The hydrogen chloride is reacted to form sodium chloride and sodium hypochlorite is formed when chlorine is absorbed by the alkaline solution. The resulting dilute hypochlorite solutions cannot be utilized and have to be disposed with the attendant high expenses in disposal cost, equipment and labor.
In the search for other possibilities of removing chlorine from the off-gases, attempts were made to utilize the known reaction of chlorine with a ferrous chloride-containing aqueous solution. A method is known from AT-B-260 643 to remove chlorine from off-gases by scrubbing the off-gas with spent hydrochloric pickling solution containing up to 300 g FeCl.sub.2 /liter and up to 100 g HCl/liter. This method has the disadvantage of the reaction being rather slow so that an appropriate removal of chlorine from a large volume of off-gas requires a high expense in equipment. In order to accelerate the reaction of chlorine with the ferrous chloride of a ferrous chloride-containing solution, DE-A-25 45 342 proposed to have this reaction take place in the presence of copper ions. Because of its copper ion content the spent solution cannot simply be reutilized. It is therefore regenerated at high cost and directly reused for the scrubbing of off-gas. Also this process requires a great deal of time if utilized on an industrial scale.